Underground storage cavern having laterally spaced well and method therefor



1958 H. v. HENDRIX UNDERGROUND STORAGE CAVERN HAVING LATERALLY SPACEDWELL AND METHOD THEREFOR Filed Dec. 28, 1953 an 550 r mm IN V EN TOR. flI/lfendzix BYM MAY sx/sr co7v-5cr0 7'0 COMMON OUTLET.

D/RA-C TlO/VALL Y DIP/LED H045 AND CA VERN. DUPL ICA TES United StatesPatent "ice UNDERGRGUND STORAGE CAVERN HAVING LATERALLY SPACE!) WELL ANDMETHOD THEREFOR Hurshel V. Hendrix, Bartlesville, kla., assignor toPhillips Petroleum Company, a corporation of Delaware ApplicationDecember 28, 1953, Serial No. 400,657

b Claims. (Cl. 61-.5)

This invention relates to an improved system of underground storagecaverns for volatile fluids and to a method of making them. In its mostspecific aspect it relates to the preparation of a single ormulti-cavern system for the underground storage of LPG (liquid petroleumgas).

This is effected by establishing communication between two or moreewells in a salt bed, or similar formation, by the use of directionaldrilling, followed by circulating a solvent through the Wells todissolve out one or more caverns.

A perennial problem of petroleum refiners during the summer months isthe storage of excess by-products which have no market, such as fuel oiland liquefied petroleum gas (LPG) such as liquefied propane, liquefiedbutane, and mixtures of the two. In an effort to alleviate this storageproblem, it has been a practice for some time to store liquefiedpetroleum products in underground storage caverns formed by drilling toa salt formation and dissolving out a portion of the salt to form acavern. The advantages of underground storage over above-ground storageinclude: (1) lower costs, both initial and maintenance (the cost ofunderground storage has been estimated at about that of steel tankage);(2) savings in ground space; (3) savings in the use of steel; (4)elimination of above-ground operationalhazards; and (5) bomb-proofprotection.

While sites for the preparation of underground storage caverns bysolvent dissolution have, as far as known, been limited to saltformations, it is believed that shale beds exist which are suitable forthis purpose, since many shales disintegrate in water. It may also befeasible to dissolve space out of massive limestone with hydrochloricacid. But the extensive occurrence of salt deposits makes this type offormation a favorite for the preparation of underground storage caverns.

Salt frequently occurs as strata of varying thickness at various depthsbelow the earths surface. Sometimes a number of strata of the salt mayoccur, one above the other and separated by strata of other materialsuch as shale, etc. In the past it has been customary to remove saltfrom such beds, existing a material distance below the earths surface,by drilling isolated Wells into the bed of salt in which it is desiredto form a storage cavern. The well shaft is cased down to approximatelyor slightly below the upper surface of the salt stratum, and a tube isrun down concentrically within the casing to a point near the bottom ofthe stratum. One of these serves as an inlet tube and the other as anoutlet tube. Water is then introduced into one of the tubes in such amanner as to flood the salt bed and dissolve the salt from the bed,resulting in a substantially saturated brine, and at the same timeforming a cavern. The brine formed is withdrawn through the other tubeand conducted to the surface. The water may be introduced at asufficient pressure to force the heavier brine up through the outlettube and out through the top of the well, or an air lift or a pump maybe utilized in the brine pipe. In this conventional system the brine isreused to displace the LPG from the cavern thus formed; the brine, beingsaturated, or substantially'so, will not dissolve additional salt duringthe displacement and, hence, no additional enlargement of the cavernoccurs at this time.

However, whenthe buoyant force of the brine on the insoluble stringersof the cavern walls and roof is replaced by the lower buoyant force ofthe LPG, which occurs when the brine is displaced therewith, caving ofthis material frequently occurs. And, of course, some may occur duringthe cavern formation. As a result of this caving the brine pipes or thecasing may become plugged or broken. If such caving occurs during cavernformation, the surface of the salt bed, formerly exposed to thedissolving action of the water, becomes covered up and, consequently,contact of the water with the salt is reduced to such an extent that alarger storage volume cannot be obtained. In some cases this caving maybe so extensive as to necessitate an abandonment of the cavern.

An object of the present invention is to provide an improved method offorming underground caverns for fluid storage. A further object is toprovide an improved underground storage system which will eliminate thehazard of losing pipes through cavings and Will provide a safe place toinstall a pump. A further object is to provide a method of formingunderground caverns which will effect more efiicient salt removal. Amore specific object is to develop one or more underground storagespaces in a salt bed by directional drilling to establish communicationbetween adjacent wells and dissolution of salt from all but one well toform caverns.

In accordance with the present invention, instead of pumping water intoa single shaft and removing it from the same shaft, two or more shaftsare located some distance apart and are suitably connected by a channelthrough or beneath the salt bed. Water is then introduced through one ofthe shafts to dissolve salt from the salt bed, forming brine. This brineis then withdrawn through the other shaft in any suitable manner, as bypumping.

The figure illustrates a method of forming'a cavern according to thisinvention.

The salt bed shown in the figure consists mainly of NaCl with somechlorides of potassium and magnesium interspersed with anhydritestringers and other insoluble matter. A hole 1 is formed by drillinginto the salt bed to a distance calculated to give the desired heightfor a storage cavern, whipstocking and drilling at an oblique angle togain lateral distance, then drilling a second hole 2 to intercept hole 1at the bottom. To facilitate interception a small solution cavity can beformed at the base of the directional hole, as indicated. This isaccomplished by circulating water through the drill pipe string beforeit is pulled. Hole 1 is cased at 3 and cemented from the top of the saltbed to the surface; hole 2 is cased at 4 and cemented from the surfaceto the point of intercept. Hole 1 is preferably cased as soon as thedrill reaches the top of the salt bed. Both casings may be provided withcasing shoes 7 and 8, as shown. After the drill pipe string is pulled,fresh water is circulated into hole 1, dissolving out a cavern, asindicated, and the resulting brine is passed through the directionallydrilled hole to the intercept and out at hole 2.- The circulation rateshould ideally be such that the brine will reach saturation before itreaches the vicinity of the intercept, thus avoiding enlargement of thecavity at the base of the outlet well. While the brine may be withdrawnthrough case pie 4, it is preferable to set an inner, or brine, pipe 5in hole 2 after it is cased and to withdraw the brine through this. Thisconfines the corrosive ac Patented Nov. 25,1958

3. The brine pipe can be secured by a packer set in the annular spacebetween it and the casing at the base of hole 2. g

The process is not necessarily limited to a Z-shaft arrangement since itis believed that additional directionally drilled wells can be drilled;in the manner of hole 1, described above, to intercept the base of hole2. These additional wells could then be enlarged in the manner describedabove to form caverns, i. e., fresh water could be circulated downthrough the new well and removed as brine through existing well 2.Instead of drilling a directional well first and then drilling avertical well to intercept it, this sequence can be reversed; a verticalhole can be drilled, a cavity formed at its base by circulating waterthrough the drill pipe string, and a second hole direction ally drilledto intercept the base of the vertical hole. It is possible to arrangethe channels connected with the outlet shaft in any desired manner, e.g., to radiate the channels like the spokes of a Wheel with the outletshaft at the hub position.

By effecting the removal of salt in the above manner, the most rapidsolution is effected near the point at which the water enters the saltbed and as the water travels toward the outlet its salt concentrationbecomes greater until only relatively saturated salt solution reachesthe outlet. Consequently, if any caving results from the dissolving awayof salt from beneath the stratum thereabove, this caving will take placenear the point of entry of the water and hence at a point remote fromthe removal of the brine. The solution leaving the salt bed, beingrelatively saturated, will dissolve away little or no salt around thebase of the outlet well and, accordingly, at this point the salt bedwill remain intact and minimize caving.

By operating according to the present invention a more eificient washingis effected. Since the water travels a longer distance in contact withthe salt formation, a higher saturation of the salt in the circulatingwater is obtained. Also, since the depth of the cavern is not limited bycaving difficulties, a larger storage volume is obtained.

The absence of piping from the storage cavern makes for a moredependable cavern because the difficulty of broken and/ or plugged pipewhich occurs when filling a conventional salt cavity is avoided. Thiscan occur due to insoluble stringers which fall when the water isdisplaced with the lighter stored product, thus reducing the buoyantforce on the ledge. Since, in the present invention, the water iscirculated from the open salt well up through a semi-detached wellcemented through the salt bad, no pipe is exposed to caving.

In addition to the advantages mentioned, the absence of caving in thevicinity of the outlet pipe has another important advantage. A safeplace is provided for the installation of a down hole pump since a casedhole is provided near to the lowest point in the cavity. Using thismodification, a submerged electric motor-driven pump may be placed inthe lower portion of the brine outlet Well to pump the LPG out of thecavern. The pump is protected since, as pointed out above, there is nodanger of the pipe in the brine outlet well being broken or plugged bycaving. Operating according to this modification, any stored LPG whichis trapped above the casing shoe at the lower end of the cemented casingor in underground ledges in the roof can be evacuated. This is not thecase when the cavern is evacuated by displacement with a brine solution;when brine is pumped into the base of the cavern to force the LPG outthe top, it is obvious that if there are pockets or niches in the roofor if the casing shoe is below the roof surface some LPG will be trappedby the rising brine solution and hence cannot be evacuated in that typeof system. But when the cavern is emptied of LPG by pumping it out atthe base of the cavern,- there is, of course, no problem of entrapn ientand hence more complete evacuation is possible.

in those cases where an anhydrite layer overlies the salt formationthere is no problem of the cavern roof being dissolved away, but in theabsence of such a layer it is necessary to protect the roof and the footof the casing from the solvent employed to form the cavern. To preventdissolution of the cavern roof by the fresh water circulated downwardlythrough the cavern, with resultant caving, a hydrocarbon blanket may bemaintained in the upper portion of the cavern. For example, LPG may beused; it will, of course, float on the surface of the water injectedinto the cavern being formed and thus protect the roof from the solventaction of the water. Water can be injected through a pipe extendingthrough the surface of the hydrocarbon in the cavern or can be allowedto fall through the hydrocarbon blanket.

While a specific embodiment of the invention has been shown, theinvention is not limited thereto. For example, a plurality of cavernscan be formed in the manner described, all having a common outlet. Whileonly LPG has been recited as a fluid to be stored, it is obvious thatother fluids could be stored in the caverns of this invention as long"as the cavern walls are substantially nonreactive therewith andinsoluble therein. In place of salt formations other formations whichare soluble in aqueous solution could be employed as underground storagesites. While the specific embodiment illustrates vertical shafts, theinvention is not limited to these; if the cavern is located in the sideof a clilf or mountain the shaft can be inclined or even horizontal.These possible variations are simply illustrative of a number whichcould be employed without departing from the spirit of this inventionand are therefore intended to be covered.

This case is related to Serial No. 401,608, same assignee.

I claim:

1. A method for forming an underground storage cavern in a formationwhich is substantially soluble in aqueous solvents which comprisesdrilling an inlet from the surface of the earth to a point within saidsoluble formation, drilling at an oblique angle from said point to adeeper point in said formation, said points being laterally spaced,thereby forming a channel connecting said points, drilling an outletwell to intercept said channel at said deeper point, passing aqueoussolvent into said inlet to dissolve part of the formation adjacentthereto, and removing through said outlet the resulting solution whichis substantially saturated so that its dissolving action upon reachingsaid channel is virtually exhausted.

2. The process of claim 1 wherein the underground formation is a saltbed and the solvent is water.

3. The process of forming an underground storage cavern comprisingdrilling an inlet from the surface to a point in a subterranean saltbed, drilling at an oblique angle from said point to a deeper point insaid salt bed to form a connecting channel, said points being separatedby a substantial lateral distance, providing an outlet from said deeperpoint to the surface, continuously circulating water through said inletand channel to dissolve out a cavern in the surrounding salt bed andform brine substantially percent saturated before it reaches saidchannel, and continuously removing the brine at said outlet.

4. A process for forming an underground storage cavern in a subterraneansalt bed comprising drilling an inlet well to a point within said saltbed, drilling at an oblique angle from said point to a lower point insaid bed, said points being laterally spaced, thereby forming a channelconnecting said points, drilling an outlet well to intercept saidchannel at said lower point, casing said inlet well from the top of thesalt bed to the surface of the earth, casing said outlet well from apoint near its base to the surface of the earth, continuously passingwater through said inlet well and channel to said outlet whereby thewater dissolves salt from the exposed walls of said inlet well to form acavern and become substantially 100 percent saturated before reachingsaid channel, and

5. continuously removing the water as brine through said outlet well.

5. The process of claim 4 wherein a cavity is formed at said lower pointprior to drilling the outlet well, thereby facilitating interception ofthe outlet well with said channel.

6. In an underground storage system, in combination, a plurality ofunderground caverns in a formation which is substantially soluble inaqueous solvents, individual conduit means extending from the upperportion of each cavern to the surface, said conduit means being casedand cemented to the overlying strata, a common central conduit meanssubstantially parallel to said individual conduit means, spacedlaterally therefrom, and extending from the surface to a point withinthe soluble formation substantially below the base of the caverns, saidcommon conduit means being cased and cemented to the surroundingformation along substantially its entire length, and oblique, drilledchannels establishing open communication between the lower portion ofthe caverns and the base of said second conduit means.

7. In an underground storage system, in combination, an undergroundcavern in a formation which is substantially soluble in aqueoussolvents, first conduit means extending from the upper portion of thecavern to the surface, said conduit means being cased and cemented tothe overlying strata, second conduit means substantially parallel to thefirst, spaced laterally therefrom, and extending from the surface to apoint within the soluble formation substantially deeper than the base ofthe cavern, said second conduit means being cased and cemented to thesurrounding formation along substantially its entire length, and anoblique, drilled channel establishing open communication between thelower portion of the cavern and the base of said second conduit means.

8. The structure of claim 7 wherein said caverns are filled with liquidpetroleum gas.

9. In an underground storage system, in combination, an undergroundcavern in a formation which is substantially soluble in aqueoussolvents, first conduit means extending from the upper portion of thecavern to the surface, said first conduit means being cased and cementedto the overlying strata, second conduit means substantially parallel tothe first, spaced laterally therefrom, and extending from the surface toa point within said soluble formation substantially deeper than the baseof said cavern, said second conduit means being cased and cemented tothe surrounding formation along substantially its entire length, and achannel establishing open communication between the lower portion ofsaid cavern and the base of said second conduit means, said channelhaving a cross section substantially smaller than the cross section ofsaid cavern.

References Cited in the file of this patent UNITED STATES PATENTS1,960,932 Tracy May 29, 1934 2,009,535 Trump July 30, 1935 2,161,800Cross June 13, 1939 2,251,916 Cross Aug. 12, 1941 2,659,209 Phelps Nov.17, 1953 2,661,062 Edholm Dec. 1, 1953 FOREIGN PATENTS 364,055 FranceAug. 13, 1906

